An IEEE 802.11 wireless network is one such communications network, for example.
Consider for example the communications network shown in FIG. 1. Using the multiple-access channel, a node N1 sends data to a node N2, a node N3 sends data to a node N4, and a node N5 sends data to a node N6, a node sending when the channel is free.
A contention resolution mechanism has been defined to solve problems of simultaneous access to the channel. The node seeking to send, called the source node, measures a level of interference on the channel. If the channel is considered busy, transmission is deferred. If not, and if the channel is free for a predetermined time called the distributed interframe space (DIFS), the source node may send. The source node sends a Ready To Send (RTS) message containing information on the volume of data that it is seeking to send and its transmission data rate. The receiver node responds with a Clear To Send (CTS) message, after which the source node begins to send data. Nodes in the respective coverage areas of the source and receiver nodes that are also seeking to send then wait for the time necessary to send the data at the stated data rate.
When it has received all the data sent by the source node, the receiver node sends an acknowledgement (ACK).
It is preferable to provide power control mechanisms to increase the capacity of such a network by enabling increased spatial reuse of the channel. The power at which the source node sends is then set to a level enabling correct reception of the data by the receiver node. However, a given sending data rate is characterized by a signal-to-interference ratio threshold. If the transmit power is reduced, the chosen sending data rate may not be possible. It is therefore necessary to choose a transmit power and a sending data rate conjointly.
The paper by T-S Kim et al. published in the proceedings of the MobiCom'06 conference of September 2006 entitled “Improving Spatial Reuse Through Tuning Transmit Power, Carrier Sense Threshold, and Data Rate in Multihop Wireless Networks”, proposes a mechanism for power and data rate control. In that algorithm, a source node determines in a first step a maximum power level that does not interfere with a call in progress as a function of a measured interference level and the carrier sense threshold. The source node then determines in a second step the maximum signal-to-interference ratio obtainable at the receiver node with the maximum power level determined in the first step. That requires the receiver node to send a perceived interference level to the source node. In a third step, the source node then selects a data rate Ci such that a signal-to-noise ratio threshold associated with that data rate is less than the maximum signal-to-interference ratio and such that a signal-to-noise ratio threshold associated with the immediately higher data rate is greater than said maximum. A power level is then determined as a function of the signal-to-noise ratio threshold associated with the selected data rate.
That method has the following drawbacks. It relies on particular propagation models that certainly do not reflect the real propagation models. In the first step, the maximum power level is determined for the worst case. That first step relies on the assumption that there is only a single concurrent sender node. It further relies on the receiver node sending the measured interference level, which requires information to be exchanged between the source and receiver nodes before the data is actually sent.
There is therefore a need for a technique for implementing transmit power and sending data rate control taking account of a set of concurrent sender nodes.